1OCTOBER 1997 LEE AND WILHELMSON 2387 The Numerical Simulation of Nonsupercell Tornadogenesis. Part II: Evolution of a Family of Tornadoes along a Weak Out¯ow Boundary BRUCE D. LEE* AND ROBERT B. WILHELMSON Department of Atmospheric Sciences and National Center for Supercomputing Applications, University of Illinois, Urbana, Illinois (Manuscript received 6 November 1996, in ®nal form 31 March 1997) ABSTRACT Nonsupercell tornadogenesis along a weak out¯ow boundary has been simulated using a three-dimensional moist convective cloud model. Thermodynamic conditions similar to those observed for nonsupercell tornado (NST) events of the High Plains were utilized in the model initialization. As the ensemble system of storm, out¯ow boundary, and leading edge vortices evolve, six distinct life cycle stages for the development and decay of NSTs are documented that span a period of about 35 min. Consistent with the results of Part I of this numerical study, vortex sheet dynamics exert considerable control over the out¯ow leading edge. The progression of pretornadic life cycle stages serves to concentrate vertical vorticity effectively along the out¯ow boundary in discrete misocyclone circulations aligned in a 3-km wavelength pattern. The organization of larger-scale mis- ocyclones and ultimate intensi®cation to initial tornadic intensity occurs coincident with the rapid development of deep convection overhead. The strongest members of a family of NSTs that develop in the model maintain ground-relative surface wind speeds of greater than 30 m s21 for approximately 11 min within which wind speeds meet F1 severity criteria for 6 min. The mature vortices reach the midlevels of the moist convection and display deep, rotationally induced axial downdrafts. The rapid transition to a predominant downdraft character for the storm complex and to an out¯ow dominated subcloud air mass heralds the onset of NST dissipation. The NST evolution simulated here compares very favorably to observational NST studies. The misocyclones are shown to provide an asymmetric pattern of convective forcing along the out¯ow boundary, which supports the formation of deep moist convection directly over them. Vertical vorticity in the boundary layer misocyclones is redistributed upward into the midlevels of the moist convection (;6 km) by developing deep updrafts. The mature vortices are maintained by vertical vorticity transported from a vortex sheet located along the out¯ow boundary and by vertical vorticity produced from the tilting of horizontal vorticity in the in¯ow region southeast of the NSTs. Low-level vortex stretching is the dominant vorticity tendency term as the vortices intensify to and maintain tornadic strength. No signi®cant vertical tilting of baroclinically generated horizontal vorticity was indicated even after convective downdraft-associated new out¯ow pools formed in the environment surrounding the misocyclones. These new out¯ow pools play a major role in NST intensi®cation by increasing convergence and resultant vortex stretching along the periphery of the tornadic circulations. A comparative simulation with rain production turned off, which precluded the possibility of new out¯ow devel- opment, revealed that NST intensity was roughly 25% greater in the baseline simulation. In an additional comparative simulation where no moist convection was allowed to develop, the resultant misocyclones markedly lacked the coherent organization and intensity of the misocyclones and NSTs of the baseline simulation and no NST-strength vortices developed. A six-stage ``re®ned'' model of NST development and decay is presented. 1. Introduction numerical model (Lee and Wilhelmson 1997, hereafter LW97). Misocyclone circulations, which by de®nition In Part I of this series of articles on the simulation (Fujita 1981) have diameters less than 4 km, are the of nonsupercell tornadogenesis (NSTG), misocyclone parent circulations of nonsupercell tornadoes. The sen- initiation and evolution were investigated along out¯ow sitivity of misocyclone development to variations in en- boundaries possessing signi®cant across-front horizon- vironment vertical shear, across-front horizontal shear, tal shear with a dry, nonhydrostatic, three-dimensional and ambient stability were presented in addition to an examination of the in¯uence surface friction has on mis- ocyclone evolution. *Current af®liation: Department of Earth Sciences, University of In this paper, NSTG is simulated in a moist environ- Northern Colorado, Greeley, Colorado. ment with a three-dimensional convective cloud model. Tornadogenesis is examined along a weak out¯ow boundary with thermodynamic conditions similar to that Corresponding author address: Dr. Bruce D. Lee, Department of Earth Sciences, University of Northern Colorado, 501 20th Street, often associated with observed nonsupercell tornado Greeley, CO 80639. (NST) events of the High Plains. The overarching ob- E-mail: [email protected] jective of this phase of the numerical investigation in- q1997 American Meteorological Society 2388 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 54 volves the basic understanding of the life cycle stages supercell family tornado events associated with cyclic of NSTs and their parent misocyclones in the presence tornado production occurring over several-hour periods. and in¯uence of deep moist convection. The process by In this study, a vortex is arbitrarily de®ned as an NST which lower-tropospheric misocyclones in¯uence the when its ground relative winds exceed 30 m s21 (i.e., character of the deep convection and how this deep high F0 velocity criteria; see Fujita 1981). In section 4 convection ultimately affects the intensi®cation of the an analysis of the life cycle of one particular tornado misocyclone is not well understood. Speci®cally, we is presented, which includes an examination of vorticity wish to understand how storm processes such as the redistribution, maintenance, and intensi®cation. Section creation of new out¯ow regions in¯uence the NST. Two 5 features the impact of new out¯ow from storms ini- additional motivations for this research include 1) the tiating along the out¯ow leading edge on NST inten- con®rmation of various features associated with NSTs si®cation and examines the effect of overhead deep that have only been rarely observed and 2) the veri®- moist convection on the boundary layer misocyclones. cation and re®nement of the observational model of A ``re®ned'' model of NSTG is presented in section 6 NSTG. that incorporates the results from observational studies, The reader is referred to LW97 for a review of ob- LW97, and this numerical investigation. servational, theoretical, and modeling studies relevant to NSTs and NSTG. A short summary is included here on NSTs and the conceptual model of NSTG that has 2. Model description and experimental design resulted from observational studies. The terms ``non- a. The model supercell tornado'' as given by Wakimoto and Wilson (1989, hereafter WW89) or ``nonmesocyclone tornado'' A three-dimensional, nonhydrostatic, quasi-com- as given by Brady and Szoke (1989, hereafter BS89) pressible, ®nite difference convective cloud model have been used to describe tornadoes associated with called MSTFLOW is employed to simulate NSTG along storms not displaying the prominent pretornadic mid- a weak out¯ow boundary. This model is a hybrid spin- level rotation found in supercell storms. Bluestein off of the Klemp and Wilhelmson (1978) cloud model (1985) coined the name ``landspout'' to describe certain and the COMMAS model (Wicker and Wilhelmson NSTs that have a similar visual appearance to water- 1995) and was designed to utilize the massively parallel spouts. The current ``observational model'' for NSTG Connection Machine (CM-5) at the National Center for became established as more NST cases were analyzed, Supercomputing Applications. The high speed and large most of which occurred near the National Center for memory CM-5 proved to be an ideal platform on which Atmospheric Research's observational network in north- to run the high-resolution simulations necessary for this east Colorado. Studies by Wilson (1986), WW89, and investigation (see the appendix in LW97 for code per- BS89 built the foundation for this model of NSTG. formance information on this platform). Common attributes of NST environments include 1) the The MSTFLOW model is described in the appendix; presence of a mesoscale surface boundary possessing consequently, only a broad summary is presented here. signi®cant across-front horizontal shear (i.e., vertical The Coriolis force is neglected in these experiments, in vorticity), 2) misocyclones circulations along the keeping with the high Rossby number nature of the boundary, 3) rapidly growing cumulus congestus or ¯ows being simulated. MSTFLOW utilizes a Kessler young cumulonimbus along the boundary, and 4) only cloud microphysics formulation similar to that used in weak mid- and upper-tropospheric winds. If a fortunate Klemp and Wilhelmson (1978) and Durran and Klemp juxtaposition of moist convective updraft and misocy- (1983). A diagnostic subgrid-scale mixing parameter- clone occurs, the vortex may be stretched to tornadic ization is employed (Smagorinsky 1963; Lilly 1962; intensity (see WW89's Fig. 20). The results from a com- Clark 1979). A bulk aerodynamic surface friction pa- prehensive observational dual-Doppler study by Roberts rameterization identical to that employed by Wilhelm- and Wilson (1995, hereafter RW95) of the 15 June 1988 son and Chen (1982)